In the copending application, Ser. No. 06/792,371, filed Oct. 19, 1985, entitled "Data Availability in a Restartable Data Base System" and assigned to the assignee of the present invention, there is disclosed a method for ensuring swithover in a restartable data base sysem between a backup and a degrading active processor. By transferring workload of a degraded processor to a backup processor, continuity of service of the data base system is guaranteed.
However, as will be more fully described hereinafter, even though control may have passed to the backup processor, it is possible that, after switchover, concurrent references to the same data base may still be made by both the degraded and the backup processors. To maintain data integrity of the system after switchover, there needs to be a delay in the processing of the backup until references or updates by the degraded processor are stopped.
In prior art data processing systems, concurrent access to shared data is resolved by serialization achieved by methods such as prioritization, locking, enqueuing or semaphores. While such methods preserve the order of access, they nevertheless cannot ensure data integrity because either processor may alter the data being used by the other.